This invention relates generally to power and communications systems used to provide power and communications to downhole equipment located in a borehole, and in particular to a power and communications system in which the power signal is modulated with the data to be transmitted.
Oil and gas wells are very expensive to construct, and it is advantageous to operate these wells as efficiently as possible. One way of providing for an increased efficiency in the operation of wells is to place controllable equipment, such as controllable valves, downhole in the well bore under the control of computers located on the surface. Several prior art methods have attempted to provide power and communications between the surface equipment and the downhole equipment.
Some prior art systems have placed cables in the well bore to provide power and communications to the downhole equipment. Safely and accurately placing the cables within the well bore along side of the piping structure or string is difficult and time consuming to achieve. In addition, this requires additional equipment to be used increasing the costs associated with the well. Well bores are a harsh environment, and numerous failure mechanisms exist that cause the reliability of such systems to be unacceptably low.
Several prior art systems have attempted to use wireless communications system, relying upon the inherent coaxial nature of the well bore and the piping structure or tubing string disposed within the bore. These prior art systems however, typically provide a low frequency power signal and a higher frequency data signal. These systems typically use toroidal coils or ferromagnetic choke assemblies placed on the piping structure or strings to provide a sufficiently large series impedance to the data and power signals to electrify a predefined portion of the piping structure or string. This allows downhole equipment that either is within the predefined portion, or that could be coupled to the predefined portion, to receive electrical power by coupling to the predefined portion and the casing, which is typically at ground potential. Thus, the downhole equipment can receive sufficient power for reception of the communications messages and data sent thereto. In addition, the frequency of the power signals in these systems will determine the amount of inductance required and therefore the physical requirements of the choke. Since the power signals are typically low frequency, typically in the 50 Hz. to 400 Hz. range, the size and weight of the chokes can be quite large and cumbersome. This makes these prior art systems unsuitable for multiple-completion wells where the clearance between the tubing strings is small.
In addition, the casings and piping structures used in these wells often have discontinuities that affect the characteristic impedance. These changes, and other changes as well, in the characteristic impedance can lead to multiple reflections of a signal being transmitted. This multipath propagation causes inter-symbol interference and results in an increase in the bit error rate. To compensate for this increase in the bit error rate, the symbol period must be increased to reduce the probability of a symbol being interfered with. In the prior art systems, the lengthening of the symbol period is accomplished by lowering the data rate.
Each individual oil or gas well is a unique environment unto itself. Frequencies and modulation schemes that work in one well, may not be suitable for use in other wells, even those wells located proximate thereto. Prior art systems have suffered from the inability to structure each well individually, since once systems are lowered into place, it is physically difficult, if not impossible, to remove and reconfigure them.
Therefore, it would be advantageous to provide a system for wireless communication and power distribution in a well bore that utilizes smaller choke inductors, does not inherently limit the portion of the well bore that is electrified, and provides for more robust communication signals having a better signal-to-noise-ratio. Additionally, it would be advantageous to provide for a communications system that is unaffected by multipath propagation and may be reconfigured.